The present invention relates to a receiver for receiving transmit diversity signals in a Code Division Multiple Access (CDMA) system.
The next generation of wireless communication systems are expected to provide high voice quality as compared to current mobile communication systems and to provide high bit rate data services. At the same, mobile terminals are expected to be light-weight, more power-efficient, and inexpensive. Furthermore, mobile terminals are expected to operate reliably in many types of communication systems, and in many different environments, such as urban, suburban, and rural. In other words, next generation systems are supposed to have better quality, be more power and bandwidth efficient, and be deployed in more diverse environments, yet remain affordable for widespread market acceptance.
In many ways, the design of radiocommunication systems is made more difficult by the nature of the radio propagation channel. One phenomenon which makes radiocommunications more difficult than some other forms of communication is multi-path fading. Multi-path fading is one result of multi-path propagation which exists in radiocommunication environments. In most radiocommunication systems, there is no direct line of sight between the base station and mobile terminal. The presence of buildings, trees, hills, and other objects in the environment surrounding the mobile terminal reflect and scatter radiowaves transmitted by the base station. Thus, a signal transmitted by the base station may arrive at the mobile terminal from many different directions with different propagation delays. One effect of multi-path propagation is that the various multipath components of a received signal exhibit varying degrees of distortion, particularly in phase and amplitude. The multipath components of the transmitted signal may combine in a variety of ways, causing fluctuations in signal strength. This phenomenon is known as Rayleigh fading. For example, if two reflected signals are 180xc2x0 out-of-phase with one another, the two signals will cancel each other out. In effect, the signal disappears. Other partial out-of-phase relationships among multiple copies of a received signal produce lesser reductions in received signal strength. The degree of fading will vary as the mobile moves from one location to another so that the degree of fading experienced by the mobile terminal fluctuates. Multipath fading is one of the most significant challenges faced by the communications engineer.
One countermeasure commonly used to combat multipath fading is known as diversity. The concept of diversity is relatively simple. If several replicas of a message signal are transmitted simultaneously over independently fading channels, there is a good likelihood that at least one of the received signals will not be severely degraded by fading. Even in circumstances where each replica experiences fading, the multiple replicas may be combined in such a manner to create a usable signal.
There are many forms of diversity, including frequency diversity, time diversity, and space diversity. In frequency diversity, the message signal is transmitted using different carrier frequencies that are spaced sufficiently apart from each other to provide independently fading versions of the signal. In time diversity, the same message signal is transmitted in differing time periods. In space diversity, multiple transmitting or receiving antennas are used with spacing between adjacent antennas chosen so as to assure the independence of fading events. A diversity receiver selects or combines the received signals to improve the signal-to-noise ratio at the receiver.
The present invention relates to a receiver for diversity reception in CDMA systems. The receiver receives first and second diversity signals during first and second symbol periods respectively. The first and second diversity signals represent first and second transmit symbols encoded and transmitted from two antennas according to a space-time code. During transmission, the transmitted symbols are distorted by the channel and combine with one another to form a combined received signal. The combined received signal arrives at the mobile terminals over numerous multipath propagation channels. The receiver at the mobile terminal selects a first set of multipath echoes associated with the first symbol period and selects a second set of multipath echoes associated with the second symbol period. In one embodiment, the multipath echoes are separately decoded and then combined to obtain final estimates of the transmitted symbols. In another embodiment, the first set of multipath echoes are combined in a first Rake receiver matched to the first transmit antenna to obtain a first value and are combined with a second Rake receiver matched to the second transmit antenna to obtain a second value. The second set of multipath echoes are combined with the first Rake receiver to obtain a third value and are combined with the second Rake receiver to obtain a fourth value. A decoder decodes the first, second, third, and fourth values to obtain estimates of the originally-transmitted symbols.